Technique versus Kick like a Duck
When beginners try to kick enthusiastically, they generally use every part of their body. They flap their arms, shrug their shoulders, grit their teeth, duck their head, lean backward etc. What they really need to do is kick similar to a duck swimming.
Picture a duck swimming along, minding its business. The part of the duck which you can see, that which is above the water, is perfectly relaxed and composed, cruising along undisturbed. The picture under the water is completely different. The duck's legs are churning away, working up a storm.
This is the image a kicker should try to imitate. From the waist up the upper body and arms should be relaxed and undisturbed by what the legs and hips are doing. The head should be upright, with the chin back and down, without ducking the head. The shoulders, arms, and hands should be relaxed.
The legs and hips should be relaxed, but working away. Keep in mind that the feeling is that of the hips moving the legs, not the legs moving the hips. The tail does not wag the dog. The feeling of a kick should be that of popping a wet towel.
However, no matter how hard you are working, keep the upper body as upright as possible and let it just go along for the ride. Remember, kick duck style. However, do not flap your arms when you do it, you are never going to fly.